For a rubber composition for a bead apex of a tire, so far emphasis has been placed only on increasing a complex modulus (E*) in order to enhance steering stability.
For increasing the complex modulus (E*), there is, for example, a method of using a carbon black exhibiting a large reinforcing effect in a large amount, however, the method has a problem that tan δ is increased. On the other hand, in order to lower tan δ, for example, there is a method of increasing a content of a vulcanizing agent such as sulfur. However, in the case where a large amount of vulcanizing agent is used, partial scorch of the rubber occurs easily during an extruding process due to an accelerated initial vulcanization rate. The initial vulcanization rate may be somewhat slowed down by compounding a specified amount of a synthetic rubber, and if a retarder PVI is used, processing can be carried out. Even in that case, however, productivity still remains at a low level. In addition, when the retarder PVI is added in an amount exceeding 1.0 part by weight based on 100 parts by weight of a rubber component, blooming thereof occurs during processing of the rubber, and therefore there is a limit in improvement in a vulcanization rate.
JP2002-36832A discloses a rubber composition for a bead apex which comprises specified amounts of a rubber component and a citraconimide compound. However, the rubber composition does not comprise a phenol resin and/or a modified phenol resin, and has a problem that a complex modulus (E*) of not less than 10 MPa cannot be obtained, and steering stability and responsiveness to steering are insufficient.